1. Field of the Invention
The present invention relates to a method of producing an oil seal which provides an oil-tight seal between two mechanical parts which are assembled coaxially and rotatively relatively to each other. More particularly, the invention is concerned with a method of producing an oil seal of the type described, having a screw thread groove formed in the sealing surface thereof.
2. Description of the Prior Art
FIGS. 12 to 14 illustrate a known oil seal of the type mentioned above and a known method for producing such an oil seal, which are disclosed in Japanese Patent Publication No. 60 56618/1985, and corresponding U.S. Pat. Nos. 3,857,156 and 3,929,341. Referring first to FIG. 12, an oil seal as a unit, designated generally by a reference numeral 100, has a plastic seal ring 101 the radially outer portion of which is fixed in an annular support member 102, while the radially inner portion of the seal ring 101 is bent towards the cavity in which the fluid to be sealed exists, thus forming a seal lip 103. The seal lip 103 thus formed is provided with a screw thread 104a formed on the sealing surface thereof. This oil seal 100 is produced in accordance with the following procedure. As the first step, a tubular billet 105, having predetermined inside and outside diameters, is made from a suitable resin material such as PTFE (reinforced plastic) and, after a screw thread is formed on an end surface from the inner peripheral portion of the tubular billet 105 by a screw thread forming means 104, a piece of seal ring of a desired thickness 101 is sliced and severed from the tubular billet 105 by a suitable slicing means 106, as shown in FIG. 13. Then, the seal ring 101 is deformed by a press such that its radially inner portion is bent in a desired angle out of the plane of the radially outer portion over its entire circumference, thus forming the seal lip 103, as shown in FIG. 14. More specifically, the press used in the bending work has a lower die 107 provided with a tapered surface 170a for forming the seal lip and a cooperative upper die 108. In operation, the lower and upper dies 107, 108 are brought together in the vertical direction as viewed in FIG. 14, so as to compress and bend the seal ring in the desired angle direction thereof. The seal ring is then fixed in the support member 102, thus completing the production of the oil seal.
This known production method encounters the following problems.
Firstly, it is to be pointed out that, since the bending of the seal ring 101 is effected after the formation of the screw thread 104a, the configuration and/or the depth of the screw thread 104a is varied due to elongation of the seal ring 101 caused by the bending work, making it difficult to keep the desired configuration and depth of the screw thread.
Secondly, since the sealing surface of the seal ring 101 is pressed against the tapered surface 107a of the lower die 107 by the compressive force during the bending work, the ridges of the screw thread 104a tend to be ground and collapsed so that the screw pump function fails, thus impairing the sealing performance of the oil seal.
Furthermore, the production efficiency is not very high and the production system as a whole is rendered too large in size, due to the fact that the formation of the screw thread work and the bending work are conducted in separate steps. In addition, since the radial width of the region in which the screw thread 104a is to be formed is determined by the radial movement of the screw thread forming means 104, it is necessary to further provide a control mechanism for controlling the radial movement of the screw thread forming means 104, resulting in a complicated construction of the screw thread forming means 104 as a whole.